Tempering will have more to do with final hardness than composition. I forget off the top of my head if all 5x alloys are heat treatable. 6x alloys are. Not that anybody is heat treating their chainrings. Work hardening during machining of the teeth would be your best friend. Not much difference in my book for machining of chainrings. To me, I'd pick whichever I could get cheap plate with a higher initial hardenss in (read, amount of cold rolling done in final processing.)
I notice my aluminum rings tend to "mushroom" on the tooth face where the chain roller makes contact. Aluminum too soft for singlespeed use by me. Me prefer Surly's stainless steel rings. Much harder. Much longer wearing for me.
Dunno which of those alum alloys are better compared to one another, but neither will last as long as steel. Yeah, I'm recommending steel, in case you were wondering what my point was. <ubiquitous smiley="" emoticon="">
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